HVAC systems are commonly used to control various environmental conditions within building structures including, for example, temperature, humidity, ventilation and the like. In doing so, a blower or fan is typically used to circulate the air within the building by forcing air through the HVAC system. Some HVAC systems have one or more circulation modes. For example, some HVAC systems have a fan “on” mode, where the fan is “on” continuously, regardless of whether the HVAC system is called to heat or cool the air in the building structure. A “circulate” fan mode is also sometimes provided, which typically runs the fan for a fixed period of time during each hour, such as 20 minutes each hour. These and other circulation modes may help circulate the air within the building.
In some HVAC systems, fresh air ventilation may also be provided. Fresh air ventilation has become increasingly popular, especially since new building structures have become more energy efficient and consequently more air tight. Fresh air ventilation is used to replace stale or contaminated air inside the building structure with fresh outside air. Heat exchangers are sometimes used to exchange heat between the outgoing stale or contaminated air and the incoming fresh outside air to help improve efficiency and reduce energy costs. In some cases, a certain level of fresh ventilation is provided regardless of the actual air quality in the building. For example, in some cases, fresh air ventilation is scheduled and performed for 20 minutes of every hour. However, such ventilation schedules can result in over-ventilation of a zone when a zone is unoccupied, and under-ventilation when a zone is occupied. Over-ventilation can unduly increase energy costs while under-ventilation can reduce air quality in the building.
To help improve fresh air ventilation, some HVAC systems include one or more air quality sensors to sense the air quality in the building. Fresh air is then provided when the sensed air quality falls below a minimum air quality threshold. For example, when the air quality sensor is a carbon dioxide sensor, fresh air may be provided to a zone when the sensed carbon dioxide concentration rises above a carbon dioxide threshold.
In some cases, use of multiple discrete air quality sensors spread across the various zones of a building may not allow building owners to optimize energy savings and/or air quality because of sensor drift and other sensor inaccuracies. Compounded drift and accuracy differences can lead to unwanted under ventilation and/or over ventilation in a building. Centralized sensor systems have also been developed to use a single set of air quality sensor, with all of the air quality sensors located at a central location in the building. In such systems, a vacuum pump or the like, and a relatively complex flow metering system including tubes strung between the centralized sensor system and each of the zones, are often used to transfer air samples from the each of the monitored zones to the centrally located sensor set. Such systems, however, can be expensive, complex, less reliable, difficult to install and not easily integrated with existing HVAC systems.